In general, such a misfire of fuel failing in being ignited and burned is apt to be caused during the operation of the spark ignition type of multi-cylinder internal combustion engine (hereinafter also referred to simply as an engine or an internal combustion engine) resulting from a variety of reasons such as malfunctions of electrically controlling devices and an abruptly changed ratio of the air-fuel mixture. The misfire caused in the combustion stroke of any one of engine cylinders (hereinafter also referred to simply as cylinders) assembled in the spark ignition type of multi-cylinder internal combustion engine leads to the output torque of the engine reduced as well as a crank angular velocity lowered from those generated under a normal condition with the same load. To detect the misfire, there have so far been proposed and thus known a wide range of misfire detection apparatuses each of which judges the misfire by detecting the rotational speed fluctuation of the internal combustion engine.
As one of the typical examples of a misfire detection apparatus for use in an internal combustion engine, there is disclosed and thus known a misfire detection apparatus in Japanese Patent Laid-Open Publication No. 2002-202000. The disclosed misfire detection apparatus can enhance an accuracy of misfire detection by correcting a judging threshold value for judging the misfire corresponding to the operating condition of a tumble generator valve serving as an intake valve provided in an intake air system when comparing the time required for a crankshaft to rotate from a compression top dead center to a predetermined crank angle with the judging threshold value to judge the misfire.
In addition, Japanese Patent Laid-Open Publications Nos. 05-180063, 08-270490, 04-159438, and 2000-282940 respectively disclose misfire detection apparatuses each of which is aimed to enhance the accuracy of misfire detection. For this purpose, the misfire detection apparatus disclosed in Japanese Patent Laid-Open Publication No. 05-180063 is constituted to provide a plurality of exclusive no-load maps and loaded-load maps corresponding to a type of speed shifting mechanism exemplified by an automatic transmission and a manual transmission and selecting a different judging value map corresponding to the type of speed shifting mechanism and the condition of clutch engagement. The misfire detection apparatus disclosed in Japanese Patent Laid-Open Publication No. 08-270490 is adapted to store correction values obtained by learning rotational speed deviations between engine cylinders and correction values according to the amount of intake air (i.e., load) for each engine cylinder in respective maps and then to compare the amount of rotational speed fluctuation for each combustion stroke of each of the engine cylinders after corrected by the correction values with the misfire judging value.
The misfire detection apparatus disclosed in Japanese Patent Laid-Open Publication No. 04-159438 is designed to change a calculating condition of misfire judging level corresponding to the rotational speed of the engine and the load that is an inertia weight engaged with a crankshaft which varies by a shift position and the condition of clutch engagement.
The misfire detection apparatus disclosed in Japanese Patent Laid-Open Publication No. 2000-282940 functions to change the gain of an air-fuel ratio feedback control when a roughness (i.e., combustion fluctuation) control is active to a small value relative to the gain of the air-fuel ratio feedback control when the roughness control is inactive so as to make the correction of ignition timing in a delay angle direction not greatly restricted by the correction of ignition timing in an advance angle direction in response to changes in the air-fuel ratio.